The invention relates to an apparatus for contactless determination of a deviation from a desired interval by pulsed spark discharge along a measurement path compared to a reference path.
A known generic apparatus is shown in German patent specification No. 3,513,799, FIG. 2b, wherein a measurement spark Path is formed between a high voltage electrode and an earth electrode representing the object and a reference spark path is formed between the same high voltage electrode and an earthed reference electrode. The two paths are physically separate from each other. This physical separation leads to poor coupling of these two spark paths, and consequently, to a more or less high switching hysteresis. Because this known apparatus operates metrologically with repetition frequencies in the kHz range, the remaining residual ionization of the activated gas volume cannot be completely neutralized before the next spark discharge. The consequence is a fall in the reaction voltage and hence a shift of the changeover point as a function of the interval, from which the respective other spark path becomes active. This physical separation also results in inadequate compensation of other parameters having an influence on the reaction voltage, such as temperature, pressure, flow, or type of gas. This is particularly the case when used in proximity of welding arcs, which generate intense local gradients of these parameters. Furthermore, the known apparatus operates as an approximation switch and is therefore not suitable for use wherever monitoring of interval tolerances is required.
The object of the present invention is to develop a generic apparatus which reacts largely insensitively to changing environmental influences and further permits a continuous interval resolution at least within a certain tolerance range of a desired interval.
This and other objects are achieved by positioning the electrodes which define the measurement path and the reference path in a common gas space such that they are in environmentally symmetrical gas spaces. The variation of the interval from the desired value is determined as a function of the frequency of the spark discharge occurring in one of the spark paths which is provided with a sensor within each measurement cycle compared to the total number of spark discharge tripped by a high voltage generator during each measurement cycle. A common high voltage electrode forms a measurement path with a grounded electrode representing the object and a reference path with a reference electrode fixed to the high voltage electrode at an interval corresponding to the desired interval. Alternatively, a second high voltage electrode, which is connected electrically in parallel and physically connected with the first high voltage electrode, defines the reference path with the grounded reference electrode. Preferably, the sensor is in circuit with the measurement spark path. A device may be provided to flow a medium into the common gas spaces of the measurement paths to maintain environmental symmetry of the paths.
An evaluation unit used with the electrodes includes a first counter, having input connected to an oscillator of a high voltage generator and an output connected to a decoder and to a second counter. The sensor is connected to one of the inputs of the second counter. The first counter determines the measurement cycle and detects the total number of sparks discharged during the measurement cycle. The second counter registers the frequency of the spark discharge detected by the sensor during the measurement cycle and transfers the frequency to the decoder. The decoder forms the ratio of the frequency to the total sparks and forms a signal corresponding to the deviation of the distance from the desired distance using a stored dependency function of the ratio to the distance deviation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.